An infrastructure-based wireless network typically includes a communication network with fixed and wired gateways. Many infrastructure-based wireless networks employ a mobile unit or host which communicates with a fixed base station that is coupled to a wired network. The mobile unit can move geographically while it is communicating over a wireless link to the base station. When the mobile unit moves out of range of one base station, it may connect or “handover” to a new base station and starts communicating with the wired network through the new base station.
One example of an infrastructure-based wireless network is TDMA (time division multiple access) which is a technology used in digital cellular telephone communication that divides each cellular channel into time slots in order to increase the amount of data that can be carried. TDMA works by dividing a radio frequency into time slots and then allocating slots to multiple calls. In this way, a single frequency can support multiple, simultaneous data channels. TDMA is used by Digital-American Mobile Phone Service (D-AMPS), Global System for Mobile communications (GSM), and Personal Digital Cellular (PDC) systems. However, each of these systems implements TDMA in a somewhat different manner.
In TDMA networks, a node accessing an uplink channel during an allocated time slot, must offset its transmit time, such that its transmission is received by the destination at the exact slot boundary. In a cellular deployment, multiple subscriber stations access the uplink channel towards a central base station. Each subscriber station must account for its propagation delay to the base station and must offset its transmissions so that all transmissions are aligned perfectly at slot boundaries at the base station.
In comparison to infrastructure-based wireless networks, such as cellular networks or satellite networks, ad hoc networks are self-forming networks which can operate in the absence of any fixed infrastructure, and in some cases the ad hoc network is formed entirely of mobile nodes. An ad hoc network typically includes a number of geographically-distributed, potentially mobile units, sometimes referred to as “nodes,” which are wirelessly connected to each other by one or more links (e.g., radio frequency communication channels). The nodes can communicate with each other over a wireless media without the support of an infrastructure-based or wired network. Links or connections between these nodes can change dynamically in an arbitrary manner as existing nodes move within the ad hoc network, as new nodes join or enter the ad hoc network, or as existing nodes leave or exit the ad hoc network. Because the topology of an ad hoc network can change significantly techniques are needed which can allow the ad hoc network to dynamically adjust to these changes. Due to the lack of a central controller, many network-controlling functions can be distributed among the nodes such that the nodes can self-organize and reconfigure in response to topology changes.
Institute of Electrical and Electronics Engineers (IEEE) 802.16 is a group of broadband wireless communications standards for metropolitan area networks (MANs). IEEE 802.16 provides fixed point-to-multipoint broadband wireless systems which enable multimedia applications with wireless connection and, with a range of up to thirty (30) miles, provide a viable last mile technology.
An earlier group of IEEE standards, the 802.11 specifications, provide a wireless alternative to Ethernet LANs (local area networks); 802.16 standards are expected to complement these by enabling a wireless alternative to expensive T1 links connecting offices to each other and the Internet.
IEEE 802.16 is a point-to-multipoint (PMP) system with one hop links between a base station and a subscriber station. Such network topologies severely stress link budgets at the cell boundaries and often render the subscribers at the cell boundaries incapable of communicating with the higher-order modulations that their radios can support. Pockets of poor-coverage areas are created where high data-rate communication is impossible. This in turn brings down the overall system capacity. While such coverage voids can be avoided by deploying base stations tightly, this drastically increases both the capital expenditure and operational expenditure for the network deployment.
In an 802.16 deployment, multiple subscriber stations access the uplink channel towards a central base station. Each subscriber station must account for its propagation delay to the base station and must offset its transmissions so that all transmissions are aligned perfectly at slot boundaries at the base station. This procedure is carried out in IEEE 802.16 using a collection of processes called ranging. The standard also permits the adjustment of the transmit power and frequency at the subscriber station, by way of the ranging mechanism.
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